Low rank coals, sub-bituminous, lignite and peat contain moisture between about 20 to 50 weight percent, resulting in low heat content. Therefore, low rank coals containing high moisture levels are expensive to transport which severely restricts its marketing. Unfortunately, the coal mines are located in areas with low coal demand, for instance, Wyoming and North Dakota. High moisture coal can freeze in the winter and make handling and transportation difficult. In addition, use of low heat content coal in an industrial furnace often requires derating the existing units and increasing the design capacity for the new units.
During the drying of low rank coals, coal particles tend to shrink, crumble, disintegrate, and form excess amounts of fines and dust. Excessive amounts of fines and dust degrade the product. This makes the product difficult to handle and transport. Most importantly, auto-ignition can occur during the drying operation.
Johnson et al. in U.S. Pat. No. 3,985,516 issued Oct. 12, 1976 disclose a coal drying and passivation process. In this process, particulate pyrophoric low rank coals were passivated in a vessel containing a drying zone and a subadjacent coating zone. In this process, the fluidized gas was heated to a temperature sufficiently high to vaporize the moisture in the coal but below the temperature at which the carbonaceous material or coal will devolatilize.
Therefore, what is needed is a method to simultaneously passivate and cool carbonaceous particles.